Time of Haskell

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Programming/ Haskell
| Programming Haskell

Time of Haskell

Handling timezones correctly, along with the distinction between timestamps and calendar dates is an endless source of bugs. Being precise about which exact type of time data your program takes in and spits out can completely obviate that problem.

The de-facto standard date and time library in Haskell, time, however, can be a little obtuse to get started with. Here’s a cheatsheet for the most common use cases for the time library.

Importing and using time library

Add to your package.yaml file:

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dependencies:
  - time

In modules where you need to work with date/time data:

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import Data.Time

In GHCi:

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Prelude> import Data.Time

The primitive type in the time library is UTCTime. If you need to do anything involving the current time, it will most likely involve some conversion from/to this type. As the name suggests, it’s a timestamp in the UTC timezone.

Get time resolution

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Prelude> import Data.Time
Prelude Data.Time> getTime_resolution
0.0000001s

Getting the POSIX time

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Prelude> import Data.Time.Clock.POSIX
Prelude Data.Time.Clock.POSIX> getPOSIXTime
1628354752.8489337s

Getting the current time

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Prelude> import Data.Time

Prelude Data.Time> getCurrentTime
2021-04-03 16:22:38.674402 UTC

Getting the system time

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Prelude> import Data.Time.Clock.System
Prelude Data.Time.Clock.System> getSystemTime
MkSystemTime {systemSeconds = 1628354836, systemNanoseconds = 564557100}

Working with time

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Prelude> import Data.Time
Prelude Data.Time> fromGregorian 2020 2 31
2020-02-29

Prelude Data.Time> fromGregorianValid 2020 2 31
Nothing

Prelude Data.Time> fromGregorianValid 2019 10 7
Just 2019-10-07

Prelude Data.Time> start <- getCurrentTime
Prelude Data.Time> now <- getCurrentTime
Prelude Data.Time> diffUTCTime now start
6.6531334s

Prelude Data.Time> now
2021-04-03 16:37:51.7175715 UTC
Prelude Data.Time> addUTCTime 5 now
2021-04-03 16:37:56.7175715 UTC

Formatting dates and times

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Prelude> import Data.Time
Prelude Data.Time> now <- getCurrentTime
Prelude Data.Time> now
2021-04-03 16:32:43.4961349 UTC
Prelude Data.Time> formatTime defaultTimeLocale "%_Y-%m-%dT%H:%M:%S.%q%z" now
"2021-04-03T16:32:43.496134900000+0000"